Laser scanning devices have been developed to detect particles, dust and other contamination, as well as defects, such as scratches, cracks, pits and the like, on surfaces, especially silicon wafer substrates and photoplates. Typically, these devices operate by detecting surface scattered light in a detector, such as a photomultiplier tube. One common use for these instruments is to determine the level of contamination caused by specific pieces of process equipment. A wafer substrate or other surface is first examined in a particle detector and the number of particles larger than a predetermined critical size is counted. Next, the wafer is sent through the piece of process equipment being tested and exposed to the process. After processing, the wafer substrate surface is again examined and the number of particles thereon is counted. The hope is that the difference in the two numbers obtained, one before processing and one after processing, will represent the total number of particles added by the process equipment, so that the level of contamination by each specific piece of equipment can be quantified.
Laser scanning particle detectors only detect those particles on a surface which when illuminated produce a level of scattered light such that the scattered light intensity received and measured by the photodetector is above a certain threshold intensity. Through this threshold set in the instrument, which may be a machine threshold at the limits of particle detectability or more usually a higher user specified threshold, the particle count is determined. Unfortunately, the measured light scattering intensity for any particular particle tends to vary from scan to scan due to variations in the way a particle is illuminated, nonuniformities in the light collection system, photon statistics, optical background noise, and other causes. Particles having an average measured light scattering intensity that is near threshold may or may not be detected in any given scan, depending on whether or not the actual measured light scattering intensity is above or below threshold. Detection of particles is done with a finite probability, and therefore different scans of the same wafer will likely give different particle counts. The problem is exacerbated when the user specified threshold is near the machine threshold, since photon statistics and optical background noise tend to be relatively large compared to the low levels of scattered light from very small particles. One solution might appear to be to average a large number of counts. However, in a production environment where time is valuable, it would be desirable to have a particle detection method that produces acceptably accurate counts in only a few scans.
Referring again to the process equipment testing application for particle detectors, it is often desirable to indicate not only the number of particles added by a piece of equipment but also their spatial distribution. In order to determine which particles have been added, previous systems have determined the positions of the detected particles for each measurement with reference to a coordinate system based upon the shape of a wafer. Unfortunately, determination of particle positions with respect to the wafer edge is imprecise and in some cases can cause poor correspondence between the two measurements. Apparatus for accurately positioning and aligning wafers abound. However, these can be quite complex and costly and may significantly add to the measurement time of a wafer. It would be desirable to have a method that enables the wafer surface to be examined in any position and orientation and compared with other surface examinations.
Further complicating the comparison of two particle measurements at different times is the random nature of particle detection for particles near threshold. When comparing two measurements, a particle seen in the second scan but not in the first may have been added by the process equipment, but alternately may have been present on the wafer during the first scan with a light scattering intensity below threshold. It would be desirable to be able to distinguish these two cases so that only added particles are displayed and likewise for those particles removed.
An object of the present invention is to produce a particle detection method that results in acceptable accurate particle counts in only a few scans, even for user selected thresholds corresponding to very small particles.
Another object of the invention is to produce a particle detection method which is capable of comparing two particle scans taken at different wafer orientations and positions, and which can distinguish between particles added or removed from the surface and particles present during both scans but not detected in one of the scans.